Natural killer (NK) cells are lymphocytes that provide innate immunity to infection, co-operate with dendritic cells to initiate adaptive immunity and remodel uterine tissue in reproduction. Of the many cell-surface receptors that mediate NK-cell activities, the killer cell immunoglobulin-like receptors (KIR) for polymorphic HLA class I determinants are distinguished by their variability in genotype and phenotype. Within the individual human body, KIR expression diversifies the NK cell population and its capacity to respond to diverse pathogens. In the human population KIR gene diversity individualizes aspects of immunity and reproduction, leading to the association of KIR and HLA factors with infection, autoimmunity, pregnancy syndromes and with the beneficial and detrimental immune reactions that can follow bone-marrow transplantation for leukemia. The overall goal of this research program is to understand how the genetic and functional interactions between variable HLA class I ligands and variable KIR modulate NK cell functions. This information will allow the molecular basis for disease associations to be understood and to be manipulated in ways that will either prevent disease or speed its recovery. In the current period of funding, increasingly broader and deeper knowledge of KIR genetics has exposed limitations to our knowledge of function. These will be addressed in the proposed research. Aim 1 will reassess the simple and mutually exclusive HLA-C1 and HLA-C2 specificities, which now dominate much thinking and experiment, particularly in the field of bone-marrow transplantation. Using several functional and binding assays, the specificity of the KIR2DL1, 2 and 3 receptors for C1 and C2 will be rigorously compared. Using similar methodology, Aim 2 will test the hypothesis that one KIR2DL1 allele has different function and disease association than all other KIR2DL1 alleles. Aim 3 will focus on the highly polymorphic KIR3DL1/S1 receptor that reacts with the Bw4 motifs of HLA-A and HLA-B molecules. It will test the hypothesis that different KIR3DL1 alleles have distinct preferences for HLA-Bw4 motifs. The three most common and functionally divergent KIR3DL1/S1 alleles will be compared. In a new and sensitive single-cell assay these receptors will be challenged with natural and mutant HLA class I molecules for their functional compatibility. The results will explain outstanding discrepancy and controversy in the literature and provide new illumination on the association of KIR and HLA factors in HIV disease.